A power supply of the type referred to comprises two substantially identical transistors each having two main electrodes and a control electrode; with junction-type transistors, in particular, the main electrodes are a collector and an emitter while the control electrode is a base. One main electrode of each power transistor, usually the emitter, is connected to a first terminal of a source of direct current which has a second terminal connected by way of respective circuit branches to the other main electrode of each transistor, i.e. to the collector in the specific instance here considered. Each of these circuit branches includes a primary transformer winding associated with a load-energizing secondary winding; the two primary windings are wound on a common core and may therefore be considered respective halves of a common primary.
When the two power transistors are made alternately conductive by driving pulses applied to their control electrodes (bases), the transformer secondary carries an alternating current which upon rectification and filtering gives rise to a direct load current whose magnitude depends on the energy drawn from the source during alternate half-cycles. The amount of this energy, in turn, is determined by the duration of the respective driving pulses. Thus, a desired ratio of load voltage to source voltage can be established by making the duration of each driving pulse equal to a selected fraction of a half-cycle.
If the two primary windings are part of separate output transformers with individual secondary windings, the currents generated in these secondaries can be independently filtered to produce a pair of load voltages of opposite polarities. It is also possible to utilize the pulsating currents of these secondaries as a pair of interleaved pulse trains of like or opposite polarity energizing respective loads without prior filtering.
In all these instances it is often important to equalize the waveforms alternately traversing the two primary windings which, despite the use of two substantially identical power transistors, may significantly differ from each other in the presence of driving pulses of like duration. Thus, the collector current of one transistor may not have the same average value as that of the other transistor; a reason for such a dissymmetry of the two waveforms is the so-called storage time, due to an accumulation of minority carriers, which delays the cutoff of a transistor beyond the termination of a driving pulse applied to its base. This storage time, in fact, can vary considerably from one transistor to the next.
In the case here particularly envisaged, where the two windings are part of a common output transformer, any unbalance between positive and negative half-cycles of the primary current will shift the operating range of the transformer core toward one or the other end of its hysteresis loop whereby the secondary current will be distorted.